Transformer spark plug



y 1948. A. c. WALL TRANSFORMER SPARK PLUG Filed Feb. 7, 1945 ,v 4. kw

SUPPRESSION w/r/l ,suP PRESS/0N VOLTHGE IN V EN TOR. fllem nder C? [Va/Z QZMM- Gm Patented May 4, 1948 2,441,047 TRANSFORMER SPARK PLUG "Alexander C.'Wall,' Indianapolia'IndJ,assignbr to P. R. Mallory & C0,, lncf, Indianapblis, Ind., a

corporation of Delaware Application February 7', 1945jseriairi6. sigasi scia'ims'; (c1. 315 57) quency current is in turn converted into current 2 verl'tion in combinationwith a' spark plug for internalcombustion engines;

Fig. 2 depicts a greatly enlarged fragmentary and sectional view of a portion of the transformer 6 second'arywinding;

of similar frequency and high voltage by means of a step up transformer, the output of said transformer being introduced into the spark plug of an internal combustion engine. As it is more fully set forth in the said application, the requirements to be satisfied by the step-up transformer in systems of the described character are quite exacting in view of the small dimensions of the transformer and in view in the high operating voltage and frequency. 7

It is anpbject of the present invention, to provide an improved step-up transformerwhich fully satisfies all of the requirements set forth in the foregoing. I a

It is another object of the present invention to provide a step-up transformer of novel and improved character which combines greatly reduced dimensions with suitable dielectric characteristics Whereby it is adapted to high voltage and high frequency operation for long periods of time.

It isalso within the contemplationof thein vention tO provide a novel surface treatment for the secondary or high voltagewinding of a transformer whereby a uniform distribution of dielectric stress throughout the length of the winding and substantial freedom from corona discharge and breakdown or spark over are obtained.

The invention also contemplates a high frequency step-up transformer provided with ahigh resistance surface coating for equalizing the stress distribution along the winding thereof, which. is simple in construction, small in dimensions, and is capable of giving long and troublefree service under the most difficult operating conditions.

Qther and further objects and advantages of re invention will become apparent from the following description, taken in conjunction with the accompanying drawing, in which Fig. 1 illustrates an exploded longitudinal sec.- tional view of the transformer. embodying the in- Fig. '3 'showsas'i'milan view of a portion of the secondary winding, indicating the destructive effect'of coronadischarge thereon; I a" Fig. '4 is 'a' greatly enlarge vertical sectional view 'somewhat fragmentary in character, of'the secondary winding of a transformer, having the principles of the invention incorporated therein;

Fig.5 'isan illustration of the equivalent electrical circuit ofthe structure shown in Fig. 4; and

Fig.6 'isia graph indicatingfthe voltage distribution alor'ig'tlie secondary transformer winding withand' without utilizing the principles of the invention.

Referring novvniore particularly to Fig. 1 of the drawing," be general structural characteristics of'the'h'igli oltage transformer and its cooperation with'the'spark plug are similar to those disclosed in my'a forsaid co-pending application Ser. Nd.T549,2 78. The "spark plug comprises a metall" "casing or"shell" l'l lined with a sleeve ableiris ulat ing material, such as a heatsta'bleceramic or porcelain, which cooperates with an element I3 of similar material supporting the central electrode [4 and withja'terminal button or head l"5 on' electrode M to provide a chamber l6 in' th'e bottoiii of lwhi ch the terminal I5 is exposed. Then'i etallicfcasing' or shell H is provided at one; end with external threads I! adapted to be received in a hole in the wall of an engine cylinder and at theother end with external threads, 8 for mounting thereto a connecting cable generally denoted by numeral [9.

The transformer :secondary winding 20 is wound'about' theexterior of a substantially elongated punv ilv el Z'j of s itabl insulating terial, pref erably glass of a minimum wall thickness for separation of the primary winding 22 fromthe secondary winding at 'aminimum distance,- "For efficient operation this distance between the primary and secondary windings must be quite small compared to the distance between the secondary winding and the metallic casing ll of the spark plug. The thinvial 2! preferably is made from Pyrex glass or other glass material, thin layers of which are capable of withstanding the high temperatures and voltages to which they 'are. subjected in operation of the system, in which average voltage stresses of the order of 2,000 to 3,000'vo1ts per mil may be commen and localized stresses may at times be as high as about 10,000 volts per mil.

The secondary winding 20 is arranged upon the glass cup 2! with the low tension end at 23 located adjacent the rim of the cup and the high tension end at '24 located adjacent the closed end or relatively flat bottom of the cup. Preferably the low tension end of the-secondary winding 20 is anchored directly to the surface of the glass cup 2| adjacent the rim thereof by a body or smear 25 of metallic material, such as a silver compound which may be adhered to the glass by firing, preferably tinned with solder to anchor the wire. The high tension end 24 of the winding 20 is preferably anchored in a'similar manner to the relatively flat bottom of the cu 2| by means of another smear or body 25 of similar metallic material. A small helical spring 2'5 is compressed between the smear 26 of metallic material on the bottom of the vial to which the high tension end of the winding 20 is anchored and the electrode terminal l5, thereby providing eil'ective electrical connection and a resilient meniea sli wt; T

The transformer secondary assembly including the vial 2 l and the secondary winding 20 is removably mounted in the chamber I6 with a terminal or pigtail at 23a for the low tension end adapte'd to be held in contact with the edge 28 of the casing or shell II by the connecting cable structure when a connector or sleeve nut 29 is engaged with threads I 8.

The primary winding 22 is supported and constructed in a manner to be received or socketed in the glass cup 2| with that winding spaced by the glass cup from the secondary winding 20 at a minimum distance. The preferred primary Winding construction comprises a core 30 of suitable heat-stable insulating material, having a helical groove on the circumferential surface thereof in which the wire of the primary winding is received and seated. Any insulation material, which will withstand the high temperatures encountered atthe spark plug without giving oil ases, may beused, such as inorganic materials, 0. 3., glass or Micalex, or other materials such as asbestos fllled Bakelite subjected to a sufficient prebaking period. The ends of primary winding 22 are electrically connected to the corresponding conductors of cable l9. In view of the fact that the structural details of this winding and its connections to the cable are fully disclosed in my copending application Serial No. 549,278 no further description and illustration of these will be necessary.

After the primary winding 22 has been socketed into the glass cup 2| to be magnetically coupled with the secondary 20, the resulting transformer structure is thoroughly saturated or impregnated with a suitable insulating compound to eliminate all gas or air-filled voids thereby to avoid corona effect and attendant energy loss. A body of such impregnating compound, which may be a suitable oil, is indicated at 3|. The primary winding 22 is held in assembled position socketed within the secondary winding." with both windings located within the chamber IS in the spark plug shell or casing II, by means of the internally threaded sleeve nut 29 'threadably engaged with external threads I8. The sleeve nut 29 preferably has a flange 32 engaged behind a flange 33 on a sleeve or ferrule 34 positioned about the base portion of the core 30. The outward edge of flange 33 is thus heldin'contact with the end edge 28 of the spark plug' shell or casing H to clamp the secondary terminal or pigtail 23a therebetween and to make good electrical connection between the spark plug casing or shell and the sleeve 34.

The structural features of this transformer are well adapted to the rather rigid requirements of a mounting of the transformer in the relatively small space or chamber present in a spa k p of conventional design. This mountin efiects an efficient shielding of the transformer and eliminates long leads between the transformer and spark plug electrodes whereby possibility of certain energy loss is avoided. The transformer assembly of a typical embodiment of the invention is in overall dimensions about one inch long and about three-sights of an inch in diameter and has a turn-ratio of the order of 15 to 20:1 With the secondary winding having about 400 turns of iine insulated wire, such as No. 40 or 42. When voltage stresses on the insulation separating the primary Winding 22 from the secondary winding 20 run ashigh as 2,000 to 3,000 volts per mil and may in the case of localized stresses run possibly as high as 10,000 volts per mil, there is a serious possibility of flash-over between the primary winding, which is substantially at ground potential, and the high tension end at 24 of the secondary winding 20, if a mere sleeve of separating insulation is used. The employment of the closed bottom glass cup 2! avoids any necessity of providing a long flash-over path, such as by extending any separating tube or sleeve far beyond the high tension end of the secondary winding. The relatively thin-walled glass cup or vial element of the transformer effectively eliminates possibility of flash-over while assuring a structure of minimum dimensions that is capable of efliciently standing up under such high voltage stresses and the relatively high heat of engine operation without any undue separation of the two windings, thereby assuring maximum operational efliciency.

While transformers of the described character have provided excellent performance, certain difficulties have been experienced after long and continued operation. Careful investigation of these diiiiculties has indicated that their cause lies in the presence of corona discharge in small voids of the fluid insulating medium, oil, and the like, filling out the space around the secondary winding. Similar voids or regions of reduced dielectric strength may also occur in the sleeve of ceramic material [2 separating the transformer space from the inner walls of metal shell I I. The voids may contain some gas but, of course, their dielectric strength is greatly inferior to that of a solid dielectric or of the oil or other liquid dielectric employed in the structure, giving rise to corona discharge. Local corona discharge in the voids Will cause serious nergy losses and will considerably reduce the overall efliciency of the transformer system. Even more disastrous are voids in proximity to the turns of the secondary winding, as indicated at 36 in Fig. 2, as such voids will set up corona discharge near the wires, which causes deterioration of the insulation 31 and eats away the copper conductor of the wire as this is shown in Fig. 3. Obviously, such corona conditions constitute a grave danger to the operation of the transformer, particularly in view of the very fine gauge of the secondary Winding which would be destroyed within a relatively short operating period.

I have discovered that the outstanding problem may be solved in a remarkably simple manner. In accordance with the principles of the present invention, I apply a thin layer 38 of a conducting material of high specific resistance upon the outer surface of .the secondary winding 20,.asthis; is shown in Fig. 4, in such a manner that theends.

of this layer. are .preferablydirectly; connected to the ends of-the said winding atthe conductive metallic deposits 25 and .26 provided upon glass.

cup 21. I prefer to employ a .resistance material of the type utilized in the manufacture of potentiometers comprising .a massof collodialgraphite distributedin a vehicle of. readily. drying.

character.

Satisfactory results have. been obtained by a resistance material having the. following com.- positionzj Grams Bakelite varnish 600.

Graphite 16 Carbon black V 48 Toluene thinner l 711 Methyl alcohol These ,materials, with the exception of the Bakelite varnish, are placed-ma ballniill and are milled for; 72 hours.

2 /2 hours more.

The layer of resistance material may heap.-

plied to the surface of thesecondary winding in any suitable manner, such as by spraying, ..dip-- At the end of this time the varnish is added .and the mixture ismilledior.

ping, brushing. Best results have beenobtained by providing a plurality ofsuperposed layers or coatings of the liquid resistance material by dip-.-

ping the transformer structureintoa receptacle.

containing a body of the fluid resistancematerial,

whereby after drying a strongand continuous resistance layer. will be formed .upqnthe winding,

having a resistance of. the. order. of .several.

megohms. This resistance layer wlllconstitute a resistive by-pass across the secondarywinding for the high frequency energy. but. willcause no appreciable..energyloss.in view of. the vfact that its resistance is several thousand timesv higher than the reactance of the secondary winding. It

has been foundthat the provision of. the resistance layer on the secondary windingrin accordance with my invention, constitutes-a complete solution for the outstanding problem and that it effectively prevents the production ofdetrimental.

corona discharge and the attendantrapid deterioration .of the secondary winding. Transformers of the described improved. typehave been sub-.- jected to very long periods of continuous opera.

tion underthe most severe.operatingconditions v secondary winding is not uniformbutincreases at an accelerating rate during, progress from the;

upper, grounded, end of the winding to its lower or high potential end. The actual distribution of stress on the secondary winding, prior to applying my high resistance or suppressor layer thereto, will appear from curve 39in Fig; 6. It 1 will be noted that in the upper region of the winding the. increase of voltage .per unit distance is very small and becomesyeryhigh per unit distance at the lower orhigh potential' end of the winding. Theprobable reason'of this strange behavior of the winding may befound, in certain characteristic of high frequency currents.

As' it appearsinFlg. 1, the distancebetween the' art, the distribution of dielectric stress along the secondary winding. and the inner'surface ofthe spark plug casing or shell H, which is at-ground potential, is quite small. Thus, each turn of the secondary winding Will form asmall capacitorv with the said casing, causing relatively low impedance path directly to ground. While in general the voltage induced in each turn is uniform throughout the winding while the voltage is rising, when the spark plug fires and effectively suddenly shorts the electrodes, there is impressed 1 on the transformer a voltage wave of effectively much higher frequency than the source frequency.- At this high value of frequency the inductive impedance of each turn of the secondary winding increases directly proportional to the This causes a voltage distribution similar to that:

shown. :in Fig. 39"dotted. This is confirmed by the fact that in transformers operated in such a way that no sparking occurs at the electrodes (by using high pressure on the nose of the plug, or by cutting the electrodes away) no deterioration of the winding occurs.

This undesirable condition is radically changed by the. provision of my resistance coating upon the secondary winding, as this will appear from Fig. 5'which' is the equivalent electrical circuit of the structure shown in Fig. 4. It will be noted that resistance coating 38 will constitute a direct high resistance path of low impedance relative to the transformer secondary inductive reactance,. connected across the ends of the secondary Winding 20. The individual turns of the secondaryv winding are capacitively coupled to uniformly distributed points on the resistance, as this is diagrammatically indicated by small capacitors 40 in dotted lines. Thus, by the provision of this low impedance path in parallel with the high im pedance. path. the desired uniform distribution of voltage. drop. along the winding is reestablished, and the production of very high stresses in certain regions, the cause of detrimental corona discharge, is avoided. This will clearly appear from curVe'M in Fig. 6, which is substantially a straight line, indicating the uniform distribution of voltage drop along the secondary winding, and the suppression or absence of corona discharge.

Although the present invention has been described-in connection with a preferred embodiment thereof. variations and modifications may be resorted to by those skilled in the art without departing from the principles of the present invention. Thus, the principles of the present invention may be applied with equal or similar results jto transformers other than those of high frequency ignition systems.

Instead ofproviding the resistive material in the form of a coating on the transformer winding, it canbe incorporated as part of the wire coating or insulation. In some cases, it is also possible to .employ the resistive material as the only insulation provided on the surface of bare wire prior to the winding operation whereby upon winding turns of such wire in closely spaced contacting position, the desired high resistance bypass circuitis directly obtained. I consider all of these variations and modifications as within the true spirit andscope of the present 'invention, as disclosed in the foregoing description, and defined by the appended claims.

What is claimed is: g

1. A high voltage high frequency step-up transformer comprising in combination a cylindrical insulative sleeve, a support adapted to be held within said sleeve, 2. low voltage primary winding mounted on said support, a high voltage secondary winding wound upon the outer circumferential surface of said sleeve, and a substantially continuous coating of electrically conducting material of high resistance covering the said secondary winding, said coating being connected across the secondary winding and capacitively coupled thereto thereb constituting an impedance balancing by-pass circuit therefor adapted to suppress corona discharge on the surface of said winding.

2. A high voltage step-up transformer for operation at relatively high frequencies comprising in combination a thin-walled cylindrical sleeve closed at one end thereof and constituted of insulating material, a low voltage primary winding mounted on a support which is adapted to be held within said sleeve, a high voltage secondary winding wound in a single layer on the outer surface of said sleeve and having its portion of the highest potential with respect to the primary winding at the closed end of the sleeve, and a continuous coating of resistance material on said secondary winding connected across said a transformer winding and capacitively coupled thereto thereby constituting an impedance balancing element therefor.

3. A transformer spark plug comprising in combination a conventional spark plug structure including a metallic casing having a circuit connector chamber therein and a central electrode terminal exposed in the bottorn'of the chamber, a transformer secondary winding structure mounted in the chamber and including a cup of insulating material and a secondary winding wound about the exterior of said cup with the end of the winding adjacent the cup bottom being in electrical connection with said terminal and the other end being in electrical connection with said casing, a transformer primary winding mounted in said cup, and a high-resistance coating on the outer surface of said secondary winding having its ends connected to the ends of said winding.

4. A transformer spark plug comprising in combination a spark plug structure including a metallic casing having a circuit connector chamber therein and a control electrode terminal exposed in the bottom of the chamber, a transformer secondary Winding structure mounted in the chamber, and including a'cup of insulating material and a secondary winding wound about the exterior of said cup with the end of the winding adjacent the cup bottom being in electrical connection with said terminal and the other end being in electrical connection wih said casing, a transformer primary winding mounted in said cup, a body of insulating liquid in said chamber and substantially surrounding said secondary winding, and a continuous high-resistance coating on the surface of said secondary winding having its ends connected to the ends of said winding and its intermediate portion capacitivel coupled to corresponding portions thereof, said coating providing a low-reactance suppressor circuit for the secondary winding to equalize the distribution of potential and to prevent the production of corona discharge.

5. An electrical transformer comprising in combination a support, an elongated primary winding mounted on said support and having a substantially axially arranged supply conductor connected to the far end thereof with the near end of said winding being adapted to be connected to a primary circuit substantially at ground potential, an elongated cup of relatively thin glass snugly receiving said winding, a relatively high voltage secondary winding constituted of insulated wire wound about the exterior of said cup with an end of that winding anchored to the rim of said cup for connection to a secondary circuit substantially at ground potential and with the high tension end of that winding anchored to the bottom of said cup for secondary circuit connection, and a layer of high-resistance material on said secondary winding having its ends connected to the ends of said winding and having its intermediate portions capacitively coupled to corresponding portions of said winding through the insulation of the Winding, said layer constituting a corona suppressor circuit for said winding.

6. A transformer spark plug comprising in combination a conventional spark plug structure including a metallic casing having a circuit connector chamber therein and a central electrode terminal exposed in the bottom of the chamber, secondary winding structure mounted in the chamber and including a cup of insulating material and a secondary winding constituted of insulated wire wound about the exterior of said cup with the end of the winding adjacent the cup bottom being in electrical connection with said terminal and the other end being in electrical connection with said casing, a transformer primary winding mounted in said cup, and a high-resistance coating on the outer surface of said secondary winding having its ends electrically connected to the ends of said winding ,and having its intermediate portions capacitively coupled to corresponding portions of said winding through the insulation of the winding.

7. A transformer spark plug comprising in combination a conventional spark plug structure including a casing having a circuit connector cham ber therein and a pair of electrodes defining a spark gap therebetween, a transformer secondary winding structure mounted in said chamber including a cylindrical support and a secondary winding wound about the exterior of said support, the ends of said winding being connected, respectively, to said electrodes, a transformer primary winding disposed within said cylindrical support and coupled to said secondary winding, and a high resistance coating on the outer surface of said secondary winding having its ends connected to the ends of said winding.

8. A transformer spark plug comprising in combination a conventional spark plug structure including a metallic casing having a circuit connector chamber therein, a first electrode electrically connected to said casing and a second electrode defining a spark gap with said first electrode, a transformer secondary winding structure mounted in said chamber including a cylindrical support and a secondary winding Wound about the exterior of said support, one end of said winding being connected through said casing to said first electrode and the other end of said winding being connected to said second electrode, a transformer primary winding disposed within said cylindrical support and coupled to said secondary winding, and a high-resistance coating on the outer surface of said secondary winding having its ends electrically connected to the ends of said Winding and having its intermediate portions capacitively coupled to corresponding portions of sat-id winding through the insulation of the winding.

ALEXANDER C. WALL.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Name Date Peek July 31, 1917 Number Number Number 503,041 

